Diaphragms can be used in fluid flow chambers to divide the chambers into regions of varying volume. For example, Schal U.S. Pat. No. 4,530,759, which is hereby incorporated by reference, discloses a dialysate supply machine that employs a balance chamber having rigid outer walls and a diaphragm that divides the chamber into two regions so that as one region is being filled with fresh dialysate the other is discharged of an equal amount of spent dialysate as the diaphragm moves toward one of the rigid walls. When all of the spent dialysate has been discharged from the region, the valves at inlets and outlets to the regions are switched, and the spent dialysate side is filled, as the fresh dialysate side discharges, and the diaphragm moves toward the other wall, until all fresh dialysate has been discharged, and so on.